U–Pb dating of silicic lavas, sills and syneruptive resedimented volcaniclastic deposits of the Lower Devonian Crudine Group, Hill End Trough, New South Wales

Abstract

The Hill End Trough of central‐western New South Wales was an elongate deep marine basin that existed in the Lachlan Fold Belt from the early Late Silurian to late Early Devonian. It is represented by a regionally extensive, unfossiliferous sequence of interbedded turbidites and hemipelagites of substantially silicic volcanic derivation, which passes laterally into contemporaneous shallow‐water sedimentary rocks. The Turondale and Merrions Formations of the Lower Devonian Crudine Group are two prominent volcanogenic formations in the predominantly sedimentary trough sequence. They contain a range of primary and resedimented volcanic facies suitable for U–Pb dating. These include widespread subaqueous silicic lavas and/or lava cryptodomes, and thick sequences of crystal‐rich volcaniclastic sandstone emplaced by a succession of mass‐flows that were generated by interaction between contemporaneous subaerial pyroclastic flows and the sea. Ion microprobe dating of the two volcanogenic formations by means of the commonly used SL 13 zircon standard yields ages ranging between 411.3 ± 5.1 and 404.8 ± 4.8 Ma. Normalising the data against a different zircon standard (QGNG) yields preferred slightly older mean ages that range between 413.4 ± 6.6 and 407.1 ± 6.9 Ma. These ages broadly approximate the Early Devonian age that has been historically associated with the Crudine Group. However, the biostratigraphically inferred late Lochkovian – early Emsian (mid‐Early Devonian) age for the Merrions Formation is inconsistent with the current Australian Phanerozoic Timescale, which assigns an age of 410 Ma to the Silurian–Devonian boundary, and ages of 404.5 Ma and 395.5 Ma to the base and top of the Pragian, respectively. There is, however, good agreement if the new ages are compared with the most recently published revision of the Devonian time‐scale. This suggests that the Early Devonian stage boundaries of the Australian Phanerozoic Timescale need to be revised downward. The new ages for the Merrions Formation could also provide a time point on this time‐scale for the Pragian to early Emsian, for which no data are presently available.

A review of Devonian time scales: databases, construction and new data

“…The final line represents a solution using proportioned biozones (based on Fordham's (1992) study, and that of Johnson et al (1991)), although only small discrepancies were observed with a version based on equal biozone lengths (Tucker et al 1998, p. 183). The time scale of Compston (2000b) is based on a time line fitted to (zircon-based ID-MS and SHRIMP) data from the Lower Palaeozoic section, reinterpreted data points from Tucker et al (1998) and one Lower Devonian point from Jagodzinski & Black (1999). Two of the data from Tucker et al (1998) are significantly different (Fig.…”

“…Middle Devonian series and stages are stippled for clarity 9 Tucker et al (1998) shown as filled error boxes, coded as in Fig. 2; data reinterpreted by Compston (2000b) are shown as open diamonds, with the corresponding code letter from (a); point L* is based on Jagodzinski & Black (1999;see Compston 2000b, table 4 for full discussion); the reinterpreted age of the Kalkberg bentonite (409.3 4-1.1 Ma la), not plotted by Compston (2000b, fig. 15), has a similar numerical age to L* but a different biostratigraphic age.…”

“…The Cooscrawn Tuff bed (Fig. 8, (Jagodzinski & Black 1999) are well constrained by biostratigraphy. However, age assignment is complicated in this instance by the fact that two sets of ages are quoted, consequent upon the use of different standard reference zircons.…”

“…New techniques to correct for this problem have been developed (see Compston (2000a, b) for full discussion). Other zircon reference materials that can be used (see Jagodzinski & Black (1999)) give differing results (see below).…”

“…However, age assignment is complicated in this instance by the fact that two sets of ages are quoted, consequent upon the use of different standard reference zircons. Figure 8 shows the preferred dates of Jagodzinski & Black (1999), based on standard zircon QGNG. Compston (2000b) preferred the alternative dates, because of lack of published data on zircon QGNG and a known disturbance in its isotopic system.…”